Why do we clean used water?
Wastewater is water that has already been used at home, in schools and in factories. It is dirty and it can carry germs. If we pour it into a river, people fall sick and fish die. So we clean it first.
Wastewater has three kinds of dirt: solids (things you can see), dissolved dirt (like soap and food waste in the water) and germs (too small to see).
The stages of purification
Each stage removes one kind of dirt.
- Screening (preliminary): bars or a mesh catch big litter.
- Settling (primary treatment): water rests, so heavy solids sink as sludge and oil floats.
- Biological treatment (secondary): air is blown in. Microbes grow and eat the dissolved dirt. This is the activated sludge idea. A measure of how much dirt microbes need to eat is called BOD (biochemical oxygen demand). Lower BOD means cleaner water.
- Disinfection (final): chlorine, ozone or UV light kills germs.
Some plants add a further stage to remove nutrients such as nitrogen and phosphorus. The sludge is dried and often turned into compost or biogas.
Septic tanks
A septic tank is a small underground tank for houses that are not joined to a sewer. Toilet and kitchen water flows in. Inside, three layers form: scum (light, floats), clear water in the middle, and sludge (heavy, sinks). Microbes without air slowly break down the sludge. The middle water flows out to a soak pit or drain field, where the soil cleans it more.
The tank must be emptied every few years, and strong chemicals should not be poured in because they kill the microbes.
Try it: a jar of muddy water
Stir a spoon of soil into a jar of water. Wait 30 minutes. Guess first: where will the mud go? You will see it sink to the bottom. This is the settling stage. Now pour the water through a cloth. That is the screen. Do not drink it; it still has germs.
Reusing treated water
Treated water is not for drinking unless it gets very special extra cleaning. But it is good for gardens, farms, toilet flushing, cooling machines and washing roads. This saves fresh water. Greywater (used water from baths and basins, without toilet waste) is easier to clean and reuse than blackwater (toilet water).
Key formulas and definitions
- Dirt removed (%) = (dirt in - dirt out) ÷ dirt in × 100
- Stages in order: Screen → Settle → Air (microbes) → Disinfect → Reuse
- Key terms: sludge = heavy settled dirt; BOD = oxygen microbes need to eat the dirt; effluent = water leaving the plant
- Greywater = bath, basin, laundry water; Blackwater = toilet water
Worked examples
1. A plant gets water with 200 units of dirt and sends out water with 20 units. What percent is removed?
Removed = 200 - 20 = 180. Percent = 180 ÷ 200 × 100 = 90%.
2. Name the stage that removes plastic bags.
Screening. The bars catch large floating litter.
3. Why can we not skip the air tank and just disinfect?
Disinfection kills germs but does not remove dissolved dirt. Microbes in the air tank remove that dirt, so both are needed.
4. A septic tank has three layers. Which layer is at the top and which at the bottom?
Scum floats at the top and sludge sinks to the bottom. Clear water is in the middle and flows out.
5. A hotel treats 50,000 litres a day and reuses 60% for the garden and flushing. How much fresh water is saved each day?
60% of 50,000 = 0.60 × 50,000 = 30,000 litres.
Common mistakes
- Saying treated water is always safe to drink. It is usually for reuse, not drinking.
- Mixing up the stages. Remember S-S-A-D: Screen, Settle, Air, Disinfect.
- Pouring bleach or strong cleaner into a septic tank. It kills the microbes that clean it.
- Thinking sludge is useless. It can be dried for compost or biogas.